Stream velocity is the speed of the water in the stream. Units
are distance per time (e.g., meters per second or feet per second). Stream velocity
is greatest in midstream near the surface and is slowest along the stream bed
and banks due to friction.

Hydraulic radius (HR or just R) is the ratio of the cross-sectional
area divided by the wetted perimeter. For a hypothetical stream with a rectangular
cross sectional shape (a stream with a flat bottom and vertical sides) the cross-sectional
area is simply the width multiplied by the depth (W * D). For the same hypothetical
stream the wetted perimeter would be the depth plus the width plus the depth
(W + 2D). The greater the cross-sectional area in comparison to the wetted perimeter,
the more freely flowing will the stream be because less of the water in the
stream is in proximity to the frictional bed. So as hydraulic radius increases
so will velocity (all other factors being equal).

Stream discharge is the quantity (volume) of water passing by
a given point in a certain amount of time. It is calculated as Q = V * A, where
V is the stream velocity and A is the stream's cross-sectional area. Units of
discharge are volume per time (e.g., m3/sec or million
gallons per day, mgpd).

At low velocity, especially if the stream bed is smooth, streams may exhibit
laminar flow in which all of the water molecules flow in parallel paths.
At higher velocities turbulence is introduced into the flow (turbulent
flow). The water molecules don't follow parallel paths.

Streams carry dissolved ions as dissolved load, fine clay
and silt particles as suspended load, and coarse sands and gravels
as bed load. Fine particles will only remain suspended if flow
is turbulent. In laminar flow, suspended particles will slowly settle to the
bed.

Hjulstrom's Diagram plots two curves representing 1) the minimum stream
velocity required to erode sediments of varying sizes from the stream bed, and
2) the minimum velocity required to continue to transport sediments of varying
sizes. Notice that for coarser sediments (sand and gravel) it takes just a little
higher velocity to initially erode particles than it takes to continue to transport
them. For small particles (clay and silt) considerably higer velocities are
required for erosion than for transportation because these finer particles have
cohesion resulting from electrostatic attractions. Think of how sticky wet mud
is.

Stream competence refers to the heaviest particles a stream can
carry. Stream competence depends on stream velocity (as shown on the Hjulstrom
diagram above). The faster the current, the heavier the particle that can be
transported.

Stream capacity is the maximum amount of solid load (bed and
suspended) a stream can carry. It depends on both the discharge and the velocity
(since velocity affects the competence and therefore the range of particle sizes
that may be transported).

As stream velocity and discharge increase so do competence and capacity. But
it is not a linear relationship (e.g., doubling velocity and discharge do not
simply double competence and capacity). Competence varies as approximately the
sixth power of velocity. For example, doubling the velocity results in a 64
times increase in the competence.

Capacity varies as the discharge squared or cubed. So tripling the discharge
results in a 9 to 27 times increase in the capacity.

Therefore, most of the work of streams is accomplished during
floods when stream velocity and discharge (and therefore competence and capacity)
are many times their level during low flow regimes. This work is in the form
of bed scouring (erosion), sediment transport (bed and suspended loads), and
sediment deposition.

Stream Dynamics

Perennial and Ephemeral Streams

Gaining (effluent) streams receive water from the groundwater.
In other words, a gaining stream discharges water from the water table. On the
other hand losing (influent) streams lie above the water table
(e.g., in an arid climate) and water seeps through the stream bed to recharge
the water table below. Gaining streams are perennial streams: they flow year
around. Losing streams are typically ephemeral streams: they do not flow year
round. Th. only flow when there is sufficient runoff from recent rains or spring
snowmelt. Some streams are gaining part of the year and losing part of the year
or just in particular years, as the water table drops during an extended dry
season.

Streams have two sources of water: storm charge, from overland
flow after rain events, and baseflow, supplied by groundwater.

Flood Erosion and Deposition: As flood waters rise, the slope
of the stream as it flows to its base level (e.g., the ocean or a lake) increases.
Also, as stream depth increases, the hydraulic radius increases thereby making
the stream more free flowing. Both of these factors lead to an increase in stream
velocity. The increased velocity and the increased cross-sectional area mean
that discharge increases. As discharge and velocity increase so do the stream's
competence and capacity. In the rising stages of a flood much sediment is dumped
into streams by overland flow and gully wash. This can result in some aggradation
or building up of sediments on the stream bed. However, after the flood peaks
less sediment is carried and a great deal of bed scouring (erosion) occurs.
As the flood subsides and competence and capacity decline sediments are deposited
and the stream bed aggrades again. Even though the stream bed may return to
somewhat like its pre-flood state, huge quantities of sediments have been transported
downstream. Much fine sediment has probably been deposited on the flood plain.

Stream Patterns

Meandering Streams: At a bend in a stream the water's momentum carries
the mass of the water against the outer bank. Water piles up on the outer bank
making it a little deeper and the inner bank a little shallower. The greater
depth on the outer side of the bend also leads to higher velocity at the outer
bank. The greater velocity combined with the greater inertial force on the outer
bank erodes a deepr channel. The deeper channel reinforces the velocity increase.
The inner bank remains shallower, increasing friction, thereby reducing the
velocity.

Where the depth and velocity of the water on the outer bank increase so do
the competence and capacity. Erosion occurs on the outer bank or cut bank.
Where velocity of the water on the inner bank decreases so do the competence
and capacity. Deposition occurs, leading to the formation of a point bar.
Over time, the position of the stream changes as the bend migrates in
the direction of the cut bank. As oxbow bends accentuate and migrate,
two bends can erode together forming a cutoff and leaving an oxbow
lake.

Graded Streams: Considering the longitudinal (downstream) profile of
a stream:
Where a stream flows down a steep slope velocity will increase which will result
in increased erosion. Where that stream then flows onto a gentler slope velocity
decreases and deposition will result. This process will reduce the slope of
steep stretches and increase the slope of flatter stretches resulting in a more
even slope through the course of the stream.

The ideal graded profile of a stream is concave upward: steeper near the head or beginning and flatter near the bottom or mouth of the stream. The reason for this is that in the upper reaches of a stream its discharge is smaller. As streams merge with other streams their discharge increases, their cross-sectional area increases, and their hydraulic radius increases. As one goes downstream and the stream grows in size the waters flow more freely. In the upper reaches, a small stream must be steeper to transport its sediments. The extra gravitational energy on the steeper slope is needed to overcome the frictional forces in the shallow stream. If the slope is too gentle and velocity is too slow to transport the sediments being supplied by weathering and erosion, the sediments will pile up. This increases the gradient which causes the water to flow faster which increases erosion and transport, which then reduces the gradient. In the lower reaches of a stream, where the discharge is greater, since friction is less the stream need not be so steep to transport the load. If it were steeper than needed to transport the sediments erosion would result. But this would decrease the gradient leading to a decrease in erosion.

It seems counter-intuitive but stream velocity generally doesn't decrease on
average, on the large scale from the steep headlands to the flat plains, from
the dashing moutain brook to the broad peaceful river. The broad lowland rivers
have much greater discharge and hydraulic radius. They flow much more freely
(e.g., the water doesn't have to dash around boulders in the stream). The net
result is that velocity actually increases somwhat.

Braided Stream patterns are found where there is a very large bed load
where there is either a high sediment supply or the stream lies on a loose,
unconsolidated bed of sand and gravel. In braided streams the stream does not
occupy a single channel but the flow is diverted into many separate ribbons
of water with sand bars between.

Stream Valley Evolution

Youthful Stream Valleys have steep-sloping, V-shaped valleys
and little or no flat land next to the stream channel in the valley bottom.